CSE446 Lecture 4: Distributed Systems Model

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Questions and Answers

What is the primary function of a consensus protocol in state machine replication?

To ensure the atomic broadcast of input messages. (correct)

To limit the number of nodes in a distributed system.

To initiate the recovery process of failed nodes.

To verify the output responses of the state machines.

Which statement best describes the relationship between state machines and nodes in a replicated environment?

Each node operates independently without synchronization.

State machines are irrelevant to the functioning of nodes in replication.

Nodes can evolve their states individually, but must do so in exactly the same manner. (correct)

All nodes must replicate the state machine's initial conditions.

What advantage does state machine replication provide when a node fails?

It allows remaining nodes to recover the state or data of the failed node. (correct)

It prevents any changes from being made until the node is recovered.

It guarantees that all nodes will shut down safely.

It reduces the workload on the remaining nodes.

Which of the following is NOT a characteristic of deterministic state machines in the context of state machine replication?

They can have varying outputs for the same input under different situations. Signup and view all the answers

In the context of state machine replication, a successful consensus indicates what among the nodes?

A complete agreement on all state changes. Signup and view all the answers

What is the primary purpose of consensus algorithms in distributed systems?

To ensure all nodes agree on a shared state or data Signup and view all the answers

Which of the following describes a byzantine fault in distributed systems?

Node sending conflicting information to different parts of the system Signup and view all the answers

What is a significant challenge posed by byzantine nodes in P2P networks?

They can create corrupted and misleading messages. Signup and view all the answers

In a deterministic finite automata, what does the transition function δ model?

Defined movement from one state to another based on input symbols Signup and view all the answers

What is the result of successful database replication in terms of system resilience?

Data is preserved even when one or more nodes fail Signup and view all the answers

Which of the following statements accurately describes atomic broadcast in distributed computing?

It ensures that all nodes receive messages in the same order. Signup and view all the answers

What distinguishes non-malicious byzantine faults from malicious ones?

Non-malicious faults stem from software bugs. Signup and view all the answers

Which component is NOT part of the 5-tuple defining a deterministic finite automata?

A set of interrupt signals Signup and view all the answers

In a partially/eventually synchronous network, what assumption is made about the network's behavior over time?

The network may act asynchronously for an arbitrary period, but will eventually become synchronous. Signup and view all the answers

Which of the following statements is true regarding faulty nodes in a distributed system?

Up to f nodes may fail where f is less than N/2 or N/3. Signup and view all the answers

What describes the crash failure model in a distributed system?

Nodes cease to respond entirely without prior warning. Signup and view all the answers

In the Byzantine Generals Problem, what is the role of loyal generals?

Loyal generals must decide on the same plan of action. Signup and view all the answers

What determines whether an action taken by generals is considered a good decision in the Byzantine fault model?

If loyal generals decide on the same action despite traitors. Signup and view all the answers

Which of the following scenarios exemplifies a byzantine fault?

Some generals receive different commands, creating confusion. Signup and view all the answers

What is a key characteristic of the traitorous generals in the Byzantine model?

They can lead loyal generals to make poor decisions if given the opportunity. Signup and view all the answers

In the context of fault models, what distinguishes byzantine failures from crash failures?

Byzantine failures result in nodes remaining active but behaving erratically. Signup and view all the answers

Study Notes

Course Information

Course title: CSE446: Blockchain & Cryptocurrencies
Lecture: 4 - Distributed Systems & Consensus Algorithms

Distributed System Model

A distributed system comprises multiple computers (nodes)
Nodes are geographically dispersed but interconnected via a communication network
Each node is autonomous, operating independently
Nodes communicate using the network

Distributed System Model (detailed)

Each node has:
- A processor
- Communication network
- Software
- Non-volatile storage
Each processor has volatile memory inaccessible by other nodes
Each node has a network interface card (NIC) for network connection
Software primarily consists of the operating system (OS)
Non-volatile storage stores programs and data

Distributed System Model (alternative view)

Distribution can be viewed as a logical construct from an application standpoint
Distributed applications consist of concurrently executing processes
Executing processes involves sequential programs
Concurrent processes can execute on a single processor or use a multi-programming approach
Primary focus is on parallel processes running across different nodes

Distributed System Model - Types of Concurrent Processes

Independent: Processes with disjoint sets of accessed objects
Competing: Processes sharing resources but not exchanging information
Cooperating: Processes exchanging information via shared data objects or message passing

Distributed System Model - Network Types

P2P (Peer-to-Peer) and client/server networks
Synchronous, asynchronous, partially/eventually synchronous
Synchronous network messages have bounded latency of T + ∆
Asynchronous network requires message to be eventually delivered but latency not bounded
Partially/eventually synchronous assumes eventual synchronous behavior despite periods of asynchrony

Fault Model

Different behaviors of nodes (processes) for reasons like errors and corruption (faulty nodes)
Up to f nodes (out of N total) may experience failures(N/2 or N/3)
Crash failure: Nodes temporarily fail to respond due to hardware/software without prior warning
Byzantine failure: More malicious and complex fault. Nodes send inconsistent or misleading information
Byzantine fault is generally due to a malicious actor or software bug or compromised machines

Fault Model: Byzantine Fault

Two types of nodes: loyal or traitors
Loyal nodes must agree on a plan of action
The number of malicious nodes (traitors) should be limited such that loyal nodes can still successfully arrive at a correct decision.
General 2 and General 3 received conflicting messages about the decision.

Need for Consensus Algorithm

Consensus is crucial in distributed applications, particularly database replication
Database replication involves storing data in multiple sites/nodes
Replication enhances resilience against node failures, improving data availability
Copying data from one server to another ensures that users share the same data avoids inconsistencies

State Machine

An abstract model of a digital computer
Has a defined set of states
Contains rules for transitioning between states triggered by input symbols
Can be deterministic or non-deterministic

Atomic Broadcast

In fault-tolerant systems, atomic broadcast ensures all nodes receive messages in the same order

State Machine Replication (SMR)

Replication approach that mirrors the state changes of a machine across several nodes
The state machine takes messages, performs calculations, then sends responses.
Machines can be replicated and work individually
Machines work together when one fails

State Machine Replication (protocol)

A protocol for distributed systems is necessary for atomic broadcast of messages
This ensures that all nodes are updated simultaneously and in the same order
The protocol is called a consensus protocol. This enables a shared state across all nodes.

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Description

Explore the fundamental concepts of distributed systems and consensus algorithms in this quiz. Understand the architecture of nodes, their components, and how they communicate within a network. Delve into both physical and logical views of distributed systems as you test your knowledge.